A 258-line C plugin (opentrack.xpl) that runs inside X-Plane and drives the six pilot-head-pose datarefs from head tracking data written by opentrack. It is the X-Plane-side counterpart to the "X-Plane" output protocol in opentrack; the two processes communicate through a POSIX shared-memory block, not a network socket.

Supported platforms: Linux, macOS (arm64 and x86_64), Windows.

opentrack process                              X-Plane process
┌─────────────────────────────┐                ┌─────────────────────────────┐
│  "X-Plane" output protocol   │                │  opentrack.xpl:              │
│  (proto-wine built with      │ ── POSIX ──>  │   flock()s the shm, reads 6  │
│  -DOTR_WINE_NO_WRAPPER):     │  shm_open +    │   doubles per frame, drives  │
│    writes {x,y,z,yaw,pitch,  │  mmap          │   sim/graphics/view/         │
│    roll} into shm block      │                │    pilots_head_{x,y,z,psi,   │
│    /facetracknoir-wine-shm   │                │    the,phi}                  │
└─────────────────────────────┘                └─────────────────────────────┘

The shared-memory block is named facetracknoir-wine-shm (same name used by the Wine freetrack proxy, so one plugin binary serves both use cases).

The plugin is only compiled when the X-Plane SDK path is provided via the SDK_XPLANE CMake cache variable; with an empty path it is silently skipped.

1. Download the X-Plane SDK from developer.x-plane.com/sdk/plugin-sdk-downloads/ (free, ~2 MB zipped). Current version at time of writing: XPSDK430.zip.

2. Unzip it somewhere; note the SDK/ directory inside the zip.

3. Configure the opentrack build tree with the SDK path:

   cmake -S . -B build -DSDK_XPLANE=/path/to/XPSDK430/SDK
   cmake --build build --target opentrack-xplane-plugin opentrack-proto-wine

   • opentrack-xplane-plugin → produces opentrack.xpl (the X-Plane side).
   • opentrack-proto-wine → produces opentrack-proto-wine.dylib/.so/ .dll, the "X-Plane" entry in opentrack's output dropdown.

   Both targets need SDK_XPLANE set. Without it, the output dropdown shows only non-X-Plane protocols.

The output bundle is emitted at x-plane-plugin/opentrack.xpl.

Copy opentrack.xpl into the standard X-Plane plugin layout. The subdirectory name is platform-specific:

X-Plane 12/
  Resources/
    plugins/
      opentrack/
        mac_x64/   ← macOS (arm64 and x86_64)
        64/        ← Linux / Windows
          opentrack.xpl

One-liner for a typical macOS install:

XP12=~/X-Plane\ 12
mkdir -p "$XP12/Resources/plugins/opentrack/mac_x64"
cp x-plane-plugin/opentrack.xpl "$XP12/Resources/plugins/opentrack/mac_x64/"

Verify which path X-Plane actually picked up by grepping its X-Plane 12/Log.txt for opentrack.xpl after the next launch; if both mac_x64/ and 64/ trees exist, X-Plane loads one and silently ignores the other.

On first launch X-Plane's Log.txt (in the sim's root folder) should contain a line like opentrack init complete emitted by XPluginStart. If you instead see opentrack failed to init SHM!, the plugin couldn't open /facetracknoir-wine-shm for RW — check your filesystem permissions on /dev/shm (Linux) or System Settings → Privacy sandboxing (macOS).

1. In opentrack, pick X-Plane from the Output dropdown. This is proto-wine built in no-wrapper mode; its icon is the X-Plane logo. It writes the shared-memory block the plugin reads.
2. Start opentrack tracking as usual. The plugin is passive — it just mirrors whatever is currently in the shm block to X-Plane's head-pose datarefs every flight-loop tick.

The plugin registers two X-Plane commands that you can bind to joystick buttons or keys from Settings → Joystick or Settings → Keyboard:

Both commands operate on xplm_CommandBegin, so they fire on button press (not release).

The plugin reads six doubles from the shm and writes to X-Plane:

Translation offsets (offset_{x,y,z}) are captured at plugin start and whenever translation is re-enabled; this lets the user move around their sim cockpit from a home position that matches the current seat pose.

Defined in both x-plane-plugin/plugin.c and proto-wine/wine-shm.h:

typedef struct WineSHM {
    double        data[6];    // x,y,z,yaw,pitch,roll (see conversions)
    int           gameid, gameid2;
    unsigned char table[8];
    bool          stop;
} volatile WineSHM;

The X-Plane plugin only reads data[]; the other fields are used by the Wine freetrack flavor of proto-wine and are ignored here.

Access is guarded with a POSIX flock(fd, LOCK_SH) around the read — cheap, contention-free for single-writer/single-reader.

The plugin implements the standard X-Plane SDK lifecycle. Each callback owns specific resources, and teardown happens in reverse order so Plugin Admin → Reload Plugins is safe (no dangling function pointers inside the unloaded dylib):

XPluginStop unregisters the flight-loop callback and both command handlers before freeing the SHM, so reload doesn't leave X-Plane holding pointers into the unloaded dylib.

Failure paths in shm_wrapper_init (malloc / shm_open / ftruncate / mmap) are checked individually and log the failing syscall with strerror(errno) to X-Plane's Log.txt for diagnosis. On any failure XPluginStart returns 0 and X-Plane unloads the plugin cleanly.

• No UDP fallback in the plugin. If you want to drive X-Plane from a different machine, use opentrack's UDP over network output and a different plugin on the X-Plane side. This plugin is local-only.
• Head roll can affect the field-of-view dataref on some X-Plane builds. The commented-out field_of_view_roll_deg line at the top of XPluginStart shows the older binding. If you dislike roll acting on FoV on your X-Plane version, swap the two lines and rebuild.
• Plugin is not codesigned. On macOS, X-Plane may warn about loading an unsigned plugin. Either accept the warning or ad-hoc-sign with codesign --force -s - opentrack.xpl before installing.
• volatile WineSHM is mostly cosmetic — it prevents within-thread compiler reordering but doesn't provide cross-process synchronization. The actual safety comes from the surrounding flock(LOCK_SH) / LOCK_EX pair, which the kernel orders with implicit memory barriers.

The minimum API version is set in x-plane-plugin/CMakeLists.txt:

-DXPLM200 -DXPLM210

That targets X-Plane 10.00 and newer. If you need features from a newer SDK (e.g. XPLM400 for X-Plane 12.04+ datarefs), add the corresponding definition; see the SDK's version-define documentation for the full ladder. No new defines are needed for pose tracking as used here.